1. Field of the Invention
This invention pertains to polyurethane molded parts. More specifically it relates to reinforced polyurethane molded parts derived from room temperature-stable, heat-curable polyisocyanate-polyol mixtures.
2. Description of the Prior Art
The preparation of fiber-reinforced cellular polypolyurethane molded parts or semi-finished parts from fiber-reinforced thermoplastic polyurethanes is described in numerous patents and patent applications.
As described in GB No. 1,209,243, polyurethane foam molded parts are prepared by placing an amount of an expandable mixture of organic polyisocyanates, compounds having reactive hydrogen atoms, blowing agents, and catalysts larger than is required for complete foam expansion in a temperature-controlled, closeable mold and allowing said mixture to expand and cure therein under compression.
In order to improve the mechanical properties of such molded foam parts, fibers or fiber mats through which the foam can expand to completely fill the mold as well as reinforcing structures, for example structures of metal, plastic, or plywood, are placed in the mold and are imbedded in the molded polyurethane part that is produced by the subsequent addition polymerization reaction and foam expansion. This method has not been able to be used to produce relatively large molded parts, since sufficient penetration and wetting of the inlays can only be achieved with relatively short foam travel distances with the relatively high viscosity expandable polyurethane mixture, especially with systems having relatively short cream times. Moreover, from an equipment standpoint it is relatively complicated and expensive to fill the expandable polyurethane mixture into the mold simultaneously from several openings.
In order to eliminate this disadvantage, in U.S. Pat. No. 4,298,556 a neutral ester having a boiling point higher than that of the foaming reaction temperature of the urethane mixture in amounts ranging from 5 to 30 weight percent. The purpose of this is to improve flow characteristics. This addition of an ester can adversely affect the mechanical properties of the resulting molded parts. In addition, when this method is used, a maximum of 40 parts by weight very fine fibers can be imbedded in the foam.
In GB No. 1,146,036 a process is described for preparing mold parts in which semi-finished products of fiber-reinforced thermoplastics are heated until plastic flow is achieved and then the parts are formed. This permits stable, high-strength molded parts to be produced in a simple manner. Such parts can be used in automotive and furniture manufacturing. The semi-finished product used in this process consists of smooth roll stock, sheets, or boards which can be prepared by coating textile flat planar structures, for example, glass fiber mats, with a plastic melt using an extruder. However, when this simple process is used, the fiber mat cannot be completely saturated with the thermoplastic melt when the fiber mat is coated on both sides. Cavities or voids form in the completed semi-finished product, which greatly reduces the strength of the finished products.
Such phenomena also are not completely avoided when, as in GB No. 1,306,145, open glass fiber mats are laminated with a thermoplastic or are impregnated and then hot pressed. In addition, in a continuous embodiment of this process, there is the disadvantage that the hot semi-finished product must be cooled in a special operation after being pressed. This takes a relatively long time, so that the cycle times are greatly increased.
As disclosed in GB No. 1,451,824, one or more textile planar structures heated to from 150.degree. to 300.degree. C. and a melt of a thermoplastic material are brought together and then pressed together in a compression zone, and then cooled in order to continuously produce a semi-finished product from fiber-reinforced thermoplastic materials. The disadvantage when thermoplastic materials are incorporated in a molten state in a textile planar structure, for example glass fiber mats, is that their high viscosity causes insufficient wetting of the textile planar structure, which causes defects in the form of air inclusions.
A different principle for preparing semi-finished products from fiber-reinforced thermoplastics is described in DE application No. 20 54 471. Here, the fiber mats are coated with thermoplastic dispersions. Then the water is evaporated and the semi-finished material is heated until the plastic softens. This does produce a void-free semi-finished product, but the process is very expensive since complicated drying equipment must be used to completely remove the water.
Reversibly thermal formable fiber-reinforced rigid polyurethane plastics are obtained in GB No. 1,411,948 when inorganic and/or organic fibers are incorporated in a polyurethane reaction mixture which is liquid at temperatures to 50.degree. C., said reaction mixture being prepared by the reaction of hydroxyl group-containing polyesters and/or polyethers having hydroxyl numbers from 100 to 600 with urethane, biuret, urea, allophanate, carbodiimide, uretonimine, and/or isocyanurate group-containing polyisocyanates or polyphenyl polymethylene polyisocyanates. The disadvantage in this case is that only modified polyisocyanates can be used, which not only has an effect on the mechanical properties but also makes the process more expensive.
A process for continuously making semi-finished products from fiber-reinforced thermoplastic polyurethanes in which the fiber-containing planar structure is easily and completely wetted with the reactable polyurethane mixture, is disclosed in German Patent Application No. P 32 42 089.7. In this process the polyisocoyanates, polyhydroxyl compounds, chain extenders, catalysts, as well as optionally auxiliaries and/or additives are mixed continuously, the flowable reaction mixture is applied to the fiber-containing planar structure and is allowed to react in a continuous-sheet process in a temperature controlled zone at temperatures from 60.degree. to 220.degree. C. until hardening occurs.
The preparation of polyurethane planar structures, coverings or coatings from storage-stable, heat-curable polyurethane compositions is also known. Such compounds consist, for example in DE application No. 15 70 548, of a polyhydroxyl compound, a uretidione diisocyanate which melts at over 100.degree. C., and a chain extender which melts at over 80.degree. C., whereby at least 80 percent of the total number of uretidione diisocyanate and chain extender particles must be under 30 .mu.m.
Storage-stable, heat-curable substance mixtures suitable for use as coating agents and adhesives, as well as sealing compositions, in which the polyisocyanate is present in the form of discreet particles in the polyol and is deactivated at its surface, are also described in EP application No. 62 780. In comformity with the application area, this process only produces flexible, adhesive products.
The purpose of this claimed invention is to prepare reinforced cellular or noncellular polyurethane molded parts using a commercially feasible method in a cost-effective manner such that the disadvantages cited above would be largely eliminated. The polyurethane molded parts had to contain a high percentage of reinforcing material. However, it was also important that the reinforcing material was completely wetted with the reactable, optionally expandable, polyurethane mixture before beginning to form polyurethane.
This objective is met through the use of a heat-curable single-component polyurethane formulation which is storage-stable at room temperature to produce the polyurethane molded parts.